1. Field of the Invention
The present invention relates to a pixel and a light emitting display, and more particularly, to a pixel and a light emitting display using the pixel, the pixel including a plurality of organic light emitting diodes (OLEDs) so that an aperture ratio of the light emitting display can be improved and a reverse bias voltage can be easily applied to the OLEDs.
2. Discussion of Related Art
Recently, various flat panel displays having weight and volume less than comparable cathode ray tube (CRT) displays have been developed. In particular, light emitting displays having high luminous efficiency, high brightness, wide view angle, and high response speed are in the limelight.
An organic light emitting diode (OLED) has a structure in which an emission layer that is a thin film for emitting light is positioned between a cathode electrode and an anode electrode. Electrons and holes are injected into the emission layer so that they can be recombined to generate exciters that emit light when their energies are reduced.
A light emitting diode (LED) includes an emission layer that can be formed of an organic or inorganic material. As such, the LED can be classified as either an inorganic LED or an organic LED (or OLED), depending on the type of the emission layer.
FIGS. 1A and 1B illustrate a conventional OLED. Referring to FIGS. 1A and 1B, the OLED includes an emission layer EL, a hole transfer layer HTL, and an electron transfer layer ETL formed between an anode electrode 20 and a cathode electrode 21.
The anode electrode 20 is connected to a first power source so as to supply holes to the emission layer EL. The cathode electrode 20 is connected to a second power source lower than the first power source so as to supply electrons to the emission layer EL. That is, the anode electrode 20 has positive (+) potential higher than the potential of the cathode electrode 21, and the cathode electrode 21 has negative (−) potential lower than the potential of the anode electrode 20.
The hole transfer layer HTL accelerates the holes supplied from the anode electrode 20 to supply the holes to the emission layer EL. The electron transfer layer ETL accelerates the electrons supplied from the cathode electrode 21 to supply the electrons to the emission layer EL. The holes supplied from the hole transfer layer HTL and the electrons supplied from the electron transfer layer ETL collide with the emission layer EL. At this time, the electrons and the holes are recombined with each other. Therefore, predetermined light is generated. In more detail, the emission layer EL is formed of an organic material so that, when the electrons and the holes are recombined with each other, one of red R, green G, and blue B light components is generated.
In addition, the OLED includes a hole injection layer HIL positioned between the hole transfer layer HTL and the anode electrode 20 and an electron injection layer EIL positioned between the electron transfer layer ETL and the cathode electrode 21. The hole injection layer HIL supplies the holes to the hole transfer layer HTL. The electron injection layer EIL supplies the electrons to the electron transfer layer ETL.
FIG. 2 is a circuit diagram of a part of a conventional light emitting display. Referring to FIG. 2, four pixels are adjacent to each other, and each pixel includes an OLED and a pixel circuit. The pixel circuit includes a first transistor T1, a second transistor T2, a third transistor T3, and a capacitor Cst. Each of the first, second, and third transistors T1, T2, and T3 includes a gate, a source, and a drain; and the capacitor Cst includes a first electrode and a second electrode.
Since the pixels have the same structure, only the pixel on the left top will be described in more detail. The source of the first transistor T1 is connected to a power source Vdd through a power source supply line, the drain of the first transistor T1 is connected to the source of the third transistor T3, and the gate of the first transistor T1 is connected to a node A. The node A is connected to the drain of the second transistor T2. The first transistor T1 supplies a current corresponding to a data signal to the OLED.
The source of the second transistor T2 is connected to a data line D1, the drain of the second transistor T2 is connected to the node A, and the gate of the second transistor T2 is connected to a scan line S1. The second transistor T2 applies the data signal to the node A in accordance with a scan signal applied to the gate thereof.
The source of the third transistor T3 is connected to the drain of the first transistor T1, the drain of the third transistor T3 is connected to an anode electrode of the OLED, and the gate of the third transistor T3 is connected to an emission control line E1 to respond to an emission control signal. Therefore, the third transistor T3 controls the flow of a current that flows from the first transistor T1 to the OLED in accordance with the emission control signal to control emission of the OLED.
The first electrode of the capacitor Cst is connected to the power source Vdd through the power source supply line, and the second electrode of the capacitor Cst is connected to the node A. The capacitor Cst stores charges in accordance with the data signal and applies a signal to the gate of the first transistor T1 in accordance with the stored charges for one frame so that the operation of the first transistor T1 is maintained for the one frame.
Referring back to FIG. 1B, since the voltage applied from the OLED to the anode electrode 20 is always set higher than the voltage applied to the cathode electrode 21, as illustrated in FIG. 1B, negative (−) carriers are positioned on the anode electrode 20, and positive (+) carriers are positioned on the cathode electrode 21.
Here, when the negative (−) carriers positioned on the anode electrode 20 and the positive (+) carriers positioned on the cathode electrode 21 are maintained for a long period of time, the movements of the electrons and holes that contribute to light emission are reduced so that brightness deteriorates and afterimage is generated.
In particular, the afterimage increases when the same image (for example, a still image) is displayed for a long period of time and deteriorates a display quality. When the afterimage is generated, the OLED deteriorates, and the life of the light emitting display is reduced.
Since one OLED is connected to one pixel circuit, a plurality of pixel circuits are necessary in order to emit light from a plurality of OLEDs so that a large number of the pixel circuits are needed.
Also, as illustrated in FIG. 2, since one emission control line needs to be connected to a pixel row, the aperture ratio of the light emitting display deteriorates due to the emission control line.